PC wont boot from DVD-r

the PC booted from it twice but to my avail i was using a usb keyboard and mouse. after rectifying that problem, rebooted and now the DVD drive has dissapered from the POST of the bios, the drive is visible and works perfectly inside windows XP. i have checked to make sure all cables etc are in place. just to clarrify the disk im trying to use is a official windows 7 ultimate DVD
any help would be appreciated thanks 😀
Tom